1. Field of the Invention
The invention generally relates to an image forming apparatus. Specifically, the invention relates to an image forming apparatus having a recording head to eject liquid droplets and a sub-tank to supply a liquid to the recording head, and also to a computer readable recording medium having a computer program including sets of instructions for causing a computer to implement various processes of the image forming apparatus such as an urgent printing operation to use up ink in the sub-tank, a pressure detecting process to detect the pressure inside the sub-tank, and an ink consumption measuring process to measure an consumed amount of ink.
2. Description of the Related Art
An inkjet recording apparatus is generally known as an inkjet image forming apparatus having a liquid ejecting head (inkjet head or recording head) for ejecting ink droplets. Examples of such an inkjet image forming apparatus having the inkjet head include image forming apparatuses having a function of a printer, a facsimile machine, or a plotter, or a combination of these functions. The inkjet image forming apparatus having the inkjet head is configured to eject ink droplets from the inkjet head onto a transferred sheet to form an image. The formation of the image also includes recording, printing, and imaging. There are two types of the inkjet image forming apparatus including 1) a serial type image forming apparatus in which the inkjet head ejects ink droplets onto the sheet to form an image while traveling in a main-scanning direction; and 2) a line type image forming apparatus in which the inkjet head ejects ink droplets onto the sheet to form an image without traveling. Note that the sheet is not limited to paper but includes any media including an OHP insofar as ink droplets or other liquid can be adhered to the media. Such media are also referred to as a subject recording medium or a recording medium, a recording sheet, and a recording form.
Note that in this application, the “inkjet image forming apparatus” indicates an image forming apparatus that forms an image onto media such as paper, string, fiber, fabric, leather, metal, plastic, glass, wood, and ceramics by ejecting liquid onto such media. Note also that “forming an image” or “image formation” not only indicates providing an image having some kind of meanings onto the media such as characters and symbols, but also indicates an image without having any meanings such as patterns (i.e., by simply ejecting ink droplets onto the media). Further, “ink” is not limited to those generally called “ink”, but includes the name “ink” used as a generic name for liquid capable of forming an image such as recording liquid, fixing liquid, and “liquid”. The ink in this application also indicates DNA specimens, resist, patterning material, resin, and the like. Moreover, the “image” is not limited to the image applied to a two-dimensional object but includes the image applied to a three-dimensional object and the image formed of a molded object.
In such image forming apparatuses, an image forming apparatus having a detachably attached main-tank and a sub-tank (may also called a “head-tank” or a “buffer-tank”) configured to supply ink to a recording head via the main-tank is widely known.
For example, Japanese Patent Application Laid-Open Publication No. 2007-130979 discloses an image forming apparatus capable of controlling a sub-tank to replenish a recording head with ink. The image forming apparatus includes a suctioned amount measuring unit to measure an amount of content suctioned from the sub-tank, an air amount measuring unit to measure an amount of air in the sub-tank and an ink amount measuring unit to measure an amount of ink in the sub-tank, where a negative pressure is generated by suctioning an appropriate amount of ink or air in the sub-tank.
In general, when an ink cartridge is run out (ink end status) or almost run out (ink near end status), the image forming apparatus requests a user to replace the ink cartridge. However, if the required cartridge is out of stock, the desired ink cartridge is not immediately replicable and hence printing may not be able to continue.